1. Field of the Invention
The invention is related to a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system. More specifically, the present invention relates to a control system for vane-type or similar cam phasers utilizing oil pressure to vary the crankshaft-to-camshaft phasing.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the angle between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more xe2x80x9cvane phasersxe2x80x9d on the engine camshaft (or camshafts, in a multiple-camshaft engine). In most cases, the phasers have a rotor with one or more vanes, mounted to the end of the camshaft, surrounded by a housing with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the rotor, and the chambers in the housing, as well. The housing""s outer circumference forms the sprocket, pulley or gear accepting drive force through a chain, belt or gears, usually from the camshaft, or possibly from another camshaft in a multiple-cam engine.
The phaser operates using engine oil as the working fluid, introduced into the oil chambers on either side of vanes, so as to rotate the camshaft angularly relative to the drive from the crankshaft.
Since the phasers cannot be perfectly sealed they are subject to oil loss through leakage. During normal engine operation, the oil pressure and flow generated by the engine oil pump is generally sufficient to keep the phaser full of oil and fully functional. However, when the engine is shut down, the oil can leak from the VCT mechanism, leaving the chambers filled with air that must be purged. During engine start conditions, before the engine oil pump generates oil pressure, the lack of controlling oil pressure and air in the chambers can allow the phaser to oscillate excessively due to lack of oil, producing noise and possibly damaging the mechanism. Additionally, it is desirable to have the phaser locked in a particular position while the engine is attempting to start.
One solution employed in prior art phasers is to introduce a locking pin that will lock the phaser in a specific phase angle position relative to the crankshaft when insufficient oil exists in the chambers. These locking pins are typically spring loaded to engage and are released using engine oil pressure. Therefore, when the engine is shut down and engine oil pressure reaches some predetermined low value the spring-loaded pin will engage and lock the phaser. During engine start, the pin remains engaged until the engine oil pump generates enough pressure to release the pin.
A drawback of these current locking pins is that they must be held in the released position using the lowest engine oil pressure available, to avoid locking the VCT mechanism while the engine is running. Some engines, when operating at high oil temperatures or running at low RPM, such as at idle, can only generate a low oil pressure. In addition, engines that are worn out generate an even lower oil pressure at hot idle conditions. In some engines this may be as low as 5 PSI. If the phaser lock pin is designed to release at this low oil pressure, when a cold engine first starts the locking pin may release before all of the air is sufficiently purged from the phaser. This would allow the phaser to move before it is full of oil and fully operational. Under such conditions, the phaser could oscillate.
Therefore, a locking pin is needed that releases at a higher pressure to allow the phaser to purge a sufficient amount of air during engine start-up, while still allowing the locking pin to remain released at the lower pressures available when the engine is warmed and idling.
The present invention solves the problem of not sufficiently purging the correct quantity of air from the VCT. The insufficient purging of air is the most problematic at idle, when the engine is operating at high oil temperatures and running at a low RPM. This especially is a problem in older cars, where the engines are worn out and generate an even lower oil pressure at hot idle conditions.
The present invention comprises a mechanism that causes the locking pin to release at a higher pressure than is required to hold the locking pin in the released position. The higher pressure required for release allows the phaser to purge more air before releasing the phaser to its functions. The lower pressure required to hold the locking pin allows the VCT to operate at low oil pressure conditions, such as hot idle without the locking pin engaging or partially engaging the rotor.
The invention is a locking vane phaser for a variable camshaft timing system in an internal combustion engine, in which the locking pin requires a higher oil pressure to retract on initial start-up, but remains unlocked at the lower pressures present during high temperature and/or idle operation. The locking pin is pushed back by a ball or cylinder shaped piston in the oil passage leading to the recess in which the locking pin fits. The piston has a cross-sectional area that is smaller than the locking pin cross-sectional area. When the piston is pushed into the tapered recess, the oil can pass the piston and push against the larger area of the locking pin, so that a lower pressure is needed to hold the pin back than was required to move the piston.